Internal combustion engines provide an excellent source of work in a convenient package and are a critical part of the modern economy. Many of the recent advances in the internal combustion engine relate to reducing the emissions of the engine and specifically meeting emissions regulations promulgated by government agencies such as the Environmental Protection Agency. An important development in meeting emissions regulations is the introduction of exhaust gas recirculation (EGR). EGR reduces the peak combustion temperatures of the engine, and reduces the oxygen content in the cylinder, resulting in lower oxides of nitrogen (NOx) emissions.
One requirement for the flow of EGR is that exhaust gas pressures must be higher than inlet gas pressures, or the exhaust gas will not flow to the intake as desired. Traditionally, this requires that the exhaust manifold pressure be maintained higher than the intake manifold pressure. This is undesirable, as it creates extra backpressure on the engine, and introduces work into the system that does not reach the crankshaft and reduces the efficiency of the engine. Also, the control of EGR flow rates often is achieved by the use of controlled backpressure using a turbocharger, often a variable geometry turbocharger (VGT). This causes the VGT to be chasing two parameters—both the desired work to compress inlet air and the desired exhaust manifold pressure to control the EGR flow rate. As a result, the control of the VGT is complex and sub-optimal to both EGR flow rates and intake air compression.
Combustion engines perform work through combusting hydrocarbons to create a pressure pulse generating a pressure differential across the engine, and further converting that pressure into mechanical work. Maintaining this pressure differential is essential to the efficient functioning of the engine, and therefore the introduction of backpressure into the engine is undesirable. However, many internal combustion engines use a portion of the generated pressure difference to operate an EGR system blending exhaust gas with inlet air to lower combustion temperatures, thereby reducing the formation of environmentally harmful NOx. As lower emissions are targeted and the demand for fuel efficiency and power density of combustion engines continues many designers of internal combustion engines are challenged to improve the management of pressure within the engine.